a. Field of the Invention
The present invention relates generally to fuel burning stoves, and, more particularly, to a natural draft pellet stove for heating houses and other structures.
b. Background Art
In many areas, wood burning stoves have been largely superseded by pellet stoves for heating homes, shops, and other structures. Pellet stoves combust pellet fuel, which is a compressed by-product of the forestry industry. The pellet fuel is conventionally made by grinding and processing bows, limbs, needles, leaves, and other waste products. By comparison with cord wood, the pellet fuel has the advantage of being more economical, and also much easier to handle and store owing to its comparatively fine consistency; commonly, pellet fuel is supplied in bags or is simply stored in a walled bin until use.
Although pellet fuel thus has many advantages which promote its use for home heating, it is not entirely an ideal fuel. In particular, because of their inherently high water and resin content, the pellets are notoriously difficult to keep lit, and require high combustion temperatures for proper burning. As a result, the majority of commercially available pellet stoves resort to the expediency of electric blowers to maintain combustion, and also use an electric auger to feed the pellets into the combustion area. These various electric motors, blowers, and feed mechanisms add substantially to the cost of the finished product, with the result that commercially available pellet stoves tend to be inordinately expensive, often to the point where they are unaffordable to many people in rural areas where they are most needed. Moreover, the cost of the electricity necessary for continuous running of the electric motors means that the electric bill for operating the pellet stove often exceeds what it would have cost to simply run an electric heater without any stove at all. Still further, the availability of electric service is somewhat spotty in some rural areas, and is subject to outages during periods of bad weather, rendering the stove inoperative just when heat is most needed.
Furthermore, reliance on the various electric blower and drive motors results in mechanical complexity and, therefore, lower reliability and higher maintenance costs; for example, it is not uncommon for conventional pellet stoves to suffer multiple blower and feed auger failures in a single season of continuous use. Also, even with the blowers to maintain the draft, the fire frequently dies out in convention pellet stoves, owing to the difficulty of keeping the fuel lit; when this happens, however, the feed auger typically continues to operate unabated, ending up packing the firebox full of unburned pellets, which may not only lead to substantial mechanical damage, but also necessitates a difficult and tedious cleanup operation to remove the packed fuel from the interior of the stove.
Perhaps even more seriously, the reliance on electric blowers leads to severe compromise of the thermal efficiency of conventional pellet stoves, so that many of these produce a dismal heat output for the amount of fuel which is consumed. In addition to inherently inefficient designs, this problem in part also stems from the tendency of manufacturers to use undersized/inadequate blowers and motors, both to scrimp on manufacturing and also in an effort to keep operating costs down. Still further, most conventional pellet stoves lack sufficient storage capacity to operate unattended for more than a few hours before refilling, so that they are unable to keep the dwelling warm if the owner must leave for an extended period; for example, many conventional stoves are capable of holding only about 1/4 bag of pellet fuel.
Yet another problem with conventional pellet stoves is that many of these are notorious for producing excessive creosote and smoke during operation. In part, this stems again from the inability to maintain proper drafting and combustion of the fuel at sufficiently high temperatures. Creosote buildup, which results in large part from inadequate combustion temperatures, not only impairs heat transfer of the stove, but can ultimately lead to a serious fire hazard. Furthermore, the particulates and other harmful emissions in the smoke from conventional pellet stoves can be damaging to the environment, with the result that many of these stoves must now be fitted with expensive and only partially effective catalytic convertors in an effort to meet air quality regulations.
Accordingly, there exists a need for a pellet stove which is capable of maintaining efficient combustion of pellet fuel using natural draft, and without the need for electric blowers to do this. Furthermore, there is a need for such a stove which is self-feeding, and does not require an auger or other electrically driven mechanism for feeding fuel into the combustion area. Still further, there exists a need for a pellet stove which ensures complete combustion of the pellet fuel so as to minimize particulates and other harmful emissions in its exhaust gasses. Still further, there is a need for such a stove which is thermally efficient, so as to produce an optimum output of heat per amount of fuel consumed. Still further, there exists a need for such a stove which is economical to manufacture, so as to be affordable for a larger group of consumers, and one which is mechanically simple and reliable so as to minimize operating and maintenance costs.